Doubt

Question

Manish has to travel from A to D changing buses at stops B and C enroute. The maximum waiting time at either stop can be 8 minutes each, but any time of waiting up to 8 minutes is equally likely at both the places. He can afford up to 13 minutes of total waiting time if he is to arrive at D on time. What is the probability that Manish will arrive late at D?
A) 8/13
B) 13/64
C) 119/128
D) 9/128

Comments

My approach :
Taking two bipartite graph or matrix :
i get possibilities : (6,8)(8,6)(7,7)(7,8)(8,7)(8,8)-6/64=3/32
Please let me know what i am missing?
{missing part is : inclusion of 0 waiting time and formulate accordingly.
Answer given are wrong Thanks guys!}

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won't 0 be included in waiting time ?

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There is something wrong in this question @saxena0612..Ur approach is correct ..Just include the '0' waiting time as well in both sides as rightly mentioned by @Just_bhavana..

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Yes by including 0 we'll get total 9*9 possibilities and 6 favorable cases as mentioned by @saxena0612 So required probabilty should be 6/81 = 2/27